Implantable cardiac devices provide electrical stimulation to selected chambers of the heart to treat a number of cardiac disorders. A pacemaker, for example, is a device which paces the heart with timed pacing pulses. A pacemaker may treat bradycardia where the ventricular rate is too slow. Atrio-ventricular conduction defects (i.e., AV block) and sick sinus syndrome represent the most common causes of bradycardia for which permanent pacing may be indicated. If functioning properly, the pacemaker makes up for the heart's inability to pace itself at an appropriate rhythm in order to meet metabolic demand by enforcing a minimum heart rate. Implantable devices may also be used to treat cardiac rhythms that are too fast, with either anti-tachycardia pacing or the delivery of electrical shocks to terminate atrial or ventricular fibrillation.
Implantable devices have also been developed to promote efficient pumping of blood by affecting the manner and degree to which the heart chambers contract. The heart pumps more effectively when the chambers contract in a coordinated manner, a result normally provided by the specialized conduction pathways in both the atria and the ventricles that enable the rapid conduction of excitation (i.e., depolarization) throughout the myocardium. These pathways conduct excitatory impulses from the sino-atrial node to the atrial myocardium, to the atrio-ventricular node, and thence to the ventricular myocardium to result in a coordinated contraction of both atria and both ventricles. This both synchronizes the contractions of the muscle fibers of each chamber and synchronizes the contraction of each atrium or ventricle with the contralateral atrium or ventricle. Without the synchronization afforded by the normally functioning specialized conduction pathways, the heart's pumping efficiency is greatly diminished. Pathology of these conduction pathways and other inter-ventricular or intra-ventricular conduction deficits can be a causative factor in heart failure, which refers to a clinical syndrome in which an abnormality of cardiac function causes cardiac output to fall below a level adequate to meet the metabolic demand of peripheral tissues. In order to treat these problems, implantable cardiac devices have been developed that provide appropriately timed electrical stimulation to one or more heart chambers in an attempt to improve the coordination of atrial and/or ventricular contractions, termed cardiac resynchronization therapy (CRT). Ventricular resynchronization is useful in treating heart failure because, although not directly inotropic, resynchronization can result in a more coordinated contraction of the ventricles with improved pumping efficiency and increased cardiac output. Currently, a common form of CRT applies stimulation pulses to both ventricles, either simultaneously or separated by a specified biventricular offset interval, and after a specified atrio-ventricular delay interval with respect to the detection of an intrinsic atrial contraction.
In addition to cardiac tissue, another type of excitable tissue that propagates action potentials is neural tissue. Centrally mediated reflex pathways modulate cardiac rate, contractility, and excitability. Baroreceptors and chemoreceptors in the heart, great vessels, and lungs transmit cardiac activity through vagal and sympathetic afferent fibers to the central nervous system. Some neural stimulators treat a variety of disorders, such as epilepsy, obesity, and breathing disorders. Neural stimulation has also been proposed for various cardiovascular disorders. Experimentally, neural stimulation has been shown to have a significant effect on several cardiovascular conditions, and may be used to treat hypertension, remodeling after myocardial infarction, and heart failure. Modulation of the sympathetic and parasympathetic nervous system with neural stimulation has been shown to have positive pre-clinical and clinical benefits, such as protecting the myocardium from further remodeling and from a predisposition to fatal arrhythmias following a myocardial infarction.